Plastic materials have been recently used as various optical materials because of light weight, high tenacity and easy-dyeability thereof. One of properties required for many optical materials is a high refractive index. As high refractive optical materials, there have been found a large number of episulfide compounds capable of providing optical materials having a refractive index of 1.7 or more (refer to JP 9-71580A, JP 9-110979A and JP 9-255781A). Most of these conventionally known episulfide compounds are heat-curing compounds and, therefore, extremely low in productivity and used only in limited applications. For this reason, it has been strongly demanded to provide photocurable materials which can be produced with a high productivity.
The methods for photocuring episulfide compounds are disclosed in WO 01/57113, JP 2002-047346A, JP 2002-105110, US 2003-0022956A and JP 2003-026806A. In these published patent documents, as initiators, there are disclosed radical generators, acid generators, base generators, etc. Since base catalysts exhibit a highest activity to polymerization of the episulfide compounds, it is considered that among these initiators, the base generators are most preferably used for photocuring the episulfide compounds. However, since studies on the photo-base generators are still short in history, base generators capable of exhibiting a practically usable activity have not been found until now. The photo-based generators are disclosed in “Chemical Industry”, Vol. 50, pp. 592 to 600 (1999), “J. Polym. Sci. Part A”, Vol. 39, pp. 1329 to 1341 (2001), etc.
Conventionally, the episulfide compounds are cast into a so-called mold and then polymerized and cured therein to obtain a cured product thereof. Owing to the remarkably high refractive index of these episulfide compounds, there is a strong demand for applying the episulfide compounds to coating materials for various substrates or films. However, since coating materials made of the episulfide compounds generally exhibit a poor wetting property to various substrates, it is difficult to stably form a thin film having a thickness of from several μm to several tens μm.
An object of the present invention is to provide photocurable compositions containing an episulfide compound capable of being readily cured by irradiation of ultraviolet rays and a high-refractive cured product obtained by irradiation of ultraviolet rays, as well as a coating material containing an episulfide compound capable of being readily cured by irradiation of ultraviolet rays and a high-refractive coating film obtained by irradiation of ultraviolet rays.